How much exercise do i need to maintain bone strength at 55?

Weight bearing: To keep bones strong do weight-bearing exercises like walking, jogging, or aerobics (cycling and swimming don't count) as well as strength training. National recommendations advise 150 min/wk of moderate-intensity activity, 75 min/wk of vigorous-intensity activity, or a combination of the 2, and strength training twice a week. Adding another day or 2 of strength training per week would also help.

Resistance/Gravity: To improve bone strenght, do resistance exercise and gravity based, or impact exercise(walking, jogging, calisthenics).This is the way at any age.